1. Field of the Invention
This invention relates to an apparatus and method for acquiring a high-resolution image of a person's fingerprint and more specifically to such an apparatus and method which utilizes a personal identification card with an integrated fingerprint platen that provides a high contrast fingerprint image when the identification card is placed on a direct fingerprint reader and a finger is placed on the platen.
2. Description of the Prior Art
Although there are other methods of identifying individuals, it has become readily apparent that fingerprints provide a unique and absolute means of identification that does not require cooperation from the subject.
Typically the fingerprint is acquired from the pad of the finger (or thumb) that extends from the tip of the digit to the first joint. The term finger, fingertip or fingerprint as used hereinafter is to be understood as including the thumb, thumbtip or thumbprint, respectively. Ink or an inkless reagent is applied to the person's fingertips which are then rolled or placed flat on a dean recording surface, such as a fingerprint card, to deposit the ink or reagent on the surface in a pattern corresponding to the fingerprints of the person. Such prints are often blurred or smeared as a result of the flow of the ink and the rotation, displacement and distortion of the finger.
Electronic fingerprint acquisition systems (sometimes hereafter referred to herein as "direct fingerprint readers") have been introduced in recent times to eliminate the need for ink or a chemical reagent. In such systems, the subject's fingertip is pressed against or rolled over a flat transparent (e.g. glass) platen in an optical system. Light is directed through the platen and is reflected from the fingertip into an optical path including mirrors, lens and an electronic image sensor such as a CCD imager of the type employed in video cameras. The image of the fingerprint may be digitized, analyzed, printed out, stored, transmitted or compared with an existing fingerprint or fingerprints.
Fingerprint identification is an exacting science which requires the comparison of the many minute and unique characteristics of each fingerprint, including the pattern of ridge endings and ridge bifurcations and locations and directions of these features etc. (hereafter collectively referred to as minutiae) of each person's fingerprint. The total collection of minutiae collectively comprise a "signature" uniquely identifying the owner of the fingerprint. This signature will be hereafter referred to as a "template". Comparisons between the template of an unknown fingerprint and templates of fingerprints of known persons may be accomplished manually or by an electronic system. Regardless of the image capture method and regardless of whether the fingerprints are interpreted by a trained expert or by machine, the clarity of the print is obviously of paramount importance.
Unclear prints may be caused in chemical recording systems by (1) excessive pressure between the pliable fingertip and the flat recording surface resulting in a distortion of the fingerprint, (2) an uneven distribution of coating of the ink or other chemical (3) excessive or insufficient coating of the chemical, (4) improper movement of the finger on the card or (5) improper lifting of the finger from the card after it is rolled.
Unclear prints in the prior art electronic acquisition systems may be caused by (1) lack of contrast between the ridges and valleys, (2) interference caused by latent prints on the platen, (3) dirt on the platen resulting from use or environmental conditions, (4) scratches or other defects to the platen resulting from use, vandalism or environment conditions, (5) excessive pressure between the fingertip and the flat platen, (6) poor contact between the fingertip and platen resulting in a poor reflected image, (7) distortion caused by rolling the finger over the platen, (6) poor resolution of the fingerprint image or (7) gross distortion resulting from system configuration. Whatever the cause, an unclear print cannot be as accurately read or compared with an existing print for verification or identification purposes.
A plurality of electronic fingerprint acquisition systems have been proposed in the art for improving the fingerprint capture process. Many of these systems employ a transparent optical prism upon which the finger is placed. The prism is used to separate beams of light that impinge on fingerprint ridges from beams of light that encounter air in the valleys between fingerprint ridges at the surface of the prism to which the finger is applied. Such systems are exemplified by the following U.S. Pat. Nos.: 4,876,725 [describing a fingerprint verification system for capturing a fingerprint image and optically processing it by spatially performing a Fourier transform using a card which includes a reflection hologram formed from the fingerprint of a known person--the intensity distribution or interference pattern that results indicates the presence or absence of a match]; 4,394,773 [teaching a fingerprint sensor using a flexible, plezoelectric polymer upon which surface charge distribution is rearranged in the presence of deformations in the polymer caused by fingerprint ridges--these charge distribution patterns are sensed with CCD devices]; 4,997,601 [electronic fingerprint recognition device wherein a finger placed on a platen is scanned ultrasonically by modulated sound waves which have first passed through and been modulated/filtered by a card containing an acoustic hologram of a known fingerprint--the reflected interference pattern acts as a correlation in determining the presence or absence of a match]; 4,582,985 [teaches a smart card identity verification system with an on-board platen upon which a finger of the user is placed, an on-board fiber optic scanning system an on-board matching circuitry to compare a bit pattern generated from the sensed fingerprint to stored bit patterns generated from fingerprints of persons of known identity]; 4,311,300 [teaching a flexible polymer sensor plate for fingerprint identification systems which works on the principle of total internal reflection of light rays that impinges on an air polymer interface such as is found in valleys between fingerprint ridges and escape of light rays without total internal reflection of light rays that impinge on deformations of the polymer caused by a ridge of a fingerprint]; 4,120,585 [teaching a prism based fingerprint image capture device using a compliant optical prism as a platen]; and 4,003,656 and 4,792,226 [teaching the conventional method in electronic fingerprint scanning devices of separating light rays that impinge on fingerprint ridges from rays that impinge on the air in valleys between fingerprint ridges because of the differences in indices of refraction at the respective interfaces].
Other electronic fingerprint acquisition systems of interest are described in U.S. Pat. Nos. 3,861,142 (Leventhal Feb. 4, 1975) and 4,120,858, 4,784,484, 4,258,994, 4,936,680. These patents relate to systems in which a combination finger platen and single prism is mounted in the fingerprint capture device. Additional related patents are: U.S. Pat. Nos. 4,983,846, 4,946,276, 4,933,976, 4,932,776, 4,925,800, 4,917,987, 4,784,484, 4,577,345, 4,553,837, 4,537,484, 4,429,412, 4,428,670, 4,258,994, 4,120,585, 1,103,656, 3,959,884, 3,861,142, 3,824,951).
Several systems have been proposed which do not employ a prism, but instead employ a fiber bundle. Two such systems are described in U.S. Pat. Nos. 4,785,171 (Dowling, Jr. et al. Nov. 15, 1988) and 4,932,776 (Dowling, Jr. et al. Jun. 12, 1990). In these systems, the finger is placed onto a optical fiber bundle as opposed to a prism. The finger platen is a specially prepared surface of the fiber bundle. The fiber optic bundle has an angled surface which serves as an integral platen for an electronic fingerprint acquisition system. Image formation is based upon the principles taught in U.S. Pat. Nos. 4,003,656 and 4,792,226. The finger platen is a permanent part of the fiber bundle and is an integral part of the assembly.
Regardless of the design of the assembly for capturing fingerprint images electronically, the prior art consistently employs a costly finger platen that is integrated into the device. Some designs allow for the prism and finger platen to be replaced periodically, which is necessary when the finger platen no longer provides optimum quality fingerprint images due to dirt or defects from use and/or abuse. The replacement of the prism and finger platen is costly and time consuming and may require special training and tools. Vendors often provide a replacement prism with their prior art fingerprint imaging device and some vendors offer programs to factory recondition prisms and finger platens.
The large variety of prior art devices for capturing fingerprint images all have a common problem--platen maintenance. It is a completely natural and unavoidable occurrence that oils and dirt accumulate with use of the device. Many platens have slightly tacky polymer surfaces or coatings which are designed to improve the optical contact and resulting image contrast. Environmental conditions such as blowing dust and misuse or vandalism in unattended application can also contribute to a build-up of dirt, grease or damage on the surface of the finger platen that obscures the fingerprint images.
A plurality of approaches have been employed to combat this problem. Coatings on the platen surface have been developed to improve the optical contact between the finger and the platen. Prior art units are also typically shipped from the manufacturer with solvents and materials for cleaning the platen surface and instructions for cleaning the unit regularly.
No prior art unit has completely eliminated the need for periodic maintenance of finger platens. This requirement for regular maintenance of the finger platen makes it challenging to install a prior art electronic fingerprint device in environments where it will not be attended by a service technician. With prior art systems with fixed or integral platens, a person still needs to regularly attend to the unit and clean the platen when necessary which makes these prior art systems inconvenient and expensive to used in unattended situations. Examples of typically unattended applications are: physical access control such as prisons and security checkpoints and payment systems such as automated bank tellers.
A fingerprint capture device installed outdoors is subject to weather and may also be used by people with dirty fingers. If one person with sweaty, greasy or dirty fingers uses the unit, the platen may be obscured and the unit may be rendered both unuseable and unappealing to the next person to use the unit.
Thus, there has arisen a need for a system to capture fingerprint images that minimizes platen maintenance and provides high quality fingerprint images.